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Old Jul 10, 2012, 03:10 AM
Firepower
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North Brisbane, Australia
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I have two questions and I am using a 200mw 5.8ghz video with a CL or SPW on the plane:

1. Is there such a thing as the best number of coils for the Helical? I see 3, 5 and 6 or 7 and probably more.

2. I have seen many ways of connecting the coil to the wire or a plug and it is not clear - (well to me). But is it as simple as in Post 971 above. The shielded wire is soldered to the copper base plate and the inner wire is connected to the coil and there is no connection between them. In other words the coil is totally isolated from the copper base. (guess it is obvious but confirmation would be appreciated).

Thanks
Col
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Old Jul 10, 2012, 03:23 AM
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Quote:
Originally Posted by Firepower View Post
I have two questions and I am using a 200mw 5.8ghz video with a CL or SPW on the plane:

1. Is there such a thing as the best number of coils for the Helical? I see 3, 5 and 6 or 7 and probably more.

2. I have seen many ways of connecting the coil to the wire or a plug and it is not clear - (well to me). But is it as simple as in Post 971 above. The shielded wire is soldered to the copper base plate and the inner wire is connected to the coil and there is no connection between them. In other words the coil is totally isolated from the copper base. (guess it is obvious but confirmation would be appreciated).

Thanks
Col

Regarding 1) The number of turns define the directivity of the antenna with 3 turns being the minimum, and well, any number as the maximum. HOWEVER, research showed that turns above 7 to 8 don't really buy much more gain as the efficiency drops thereafter significantly.

Regarding 2) They have to be isolated. A coper plate (or PCB) with a hole where you solder the mesh of the shielding to the plate and the center to the helix is probably the easiest aproach.

Markus
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Old Jul 10, 2012, 03:29 AM
Firepower
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North Brisbane, Australia
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Thanks for the quick reply and explanations.

It seems quite clear in the recent photo but I have seen where a piece is cut out of the copper plate and the coil is soldered on to the copper which did not seem logical.

Your explanations clear it up.

Thanks again.
Col

Quote:
Originally Posted by markus123456 View Post
Regarding 1) The number of turns define the directivity of the antenna with 3 turns being the minimum, and well, any number as the maximum. HOWEVER, research showed that turns above 7 to 8 don't really buy much more gain as the efficiency drops thereafter significantly.

Regarding 2) They have to be isolated. A coper plate (or PCB) with a hole where you solder the mesh of the shielding to the plate and the center to the helix is probably the easiest aproach.

Markus
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Old Jul 10, 2012, 06:39 AM
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Australia, QLD, Brisbane
Joined Feb 2011
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Can anyone tell me what is the recommended distance of seperation on a pair of 4 or 5 turn 5.8Ghz helicals on a diversity set up like the one linked?

http://www.futurehobbies.com/product...ersity-package
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Old Jul 10, 2012, 07:13 AM
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Any separation of 1 wavelength or more is sufficient. It really isn't critical with our CP and diversity systems these days.

-Alex
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Old Jul 10, 2012, 07:14 AM
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Great, thanks Alex.
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Old Jul 10, 2012, 07:34 PM
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Highland, CA, USA
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Alex,

Does the circular pol;arized have a "top"?

What I am asking is I have a SP on the model, and it seems that if I rotate the helical "receiver" antenna it gets a bit better picture with the feed point about 45 degrees from the down position. Is this normal or is my helical tweaked?
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Old Jul 10, 2012, 09:08 PM
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^It depends on the match type. The Hecker transformer lends itself to these types of characteristics. Microstrip type matches (such as the Wavetrap) do not have this effect.

-Alex
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Old Jul 11, 2012, 12:08 AM
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Quote:
Originally Posted by IBCrazy View Post
^It depends on the match type. The Hecker transformer lends itself to these types of characteristics. Microstrip type matches (such as the Wavetrap) do not have this effect.
I used the "1/4 turn .5 mm gap design", is that similar to the Hecker, would it be better to change it?

I want to mount it directly to the rx, no coax, there is a perticular rotation that it lines up best with the rx and the rx mounts flat.
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Old Jul 11, 2012, 07:29 AM
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^That is a microstrip type match and will be just fine. If you had tuning equipemtn, you could do the gamma match, but I think you are jst where you need to be.

-Alex
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Old Jul 11, 2012, 10:07 AM
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"poor man" 5.8 antenna tuning

You mentioned tuning, I am doing my tuning a "poor mans" way since I don't have good equipment. Tell me what you think.

I have an milliamp meter on the tx power. I get a 20 - 40% change in milliamps between the 8 different frequencies. I have determined some of the change is that the tx is putting out more power across the range, the rest of the change is from a bad tune on the antenna.

I have several of the same transmitters and they all vary in the amp draw across the frequency range. The change is always increasing as the frequency rises regardless of antenna used.

Across the room I have a reciever with no antenna and a rssi tap . While watching the rssi voltage I can tune the tx antenna for the lowest tx amps and highest rx rssi.

When doing this the tx, rx and antenna, can't be moved any, or it affects the numbers, even my position in the room will make a difference. While making changes to the antenna the results (tx amp draw and rssi) are very repeatable so I feel i am on the right track. Flight testing has verified the results.
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Old Jul 13, 2012, 08:37 PM
Firepower
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North Brisbane, Australia
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Just wondered what the range in the diameter of the coil that is the best.

The first one I made should be all right and have now cut the end of the coil correctly.

I have just made another coil on round balsa but the inside diameter is 12.5mm and the outside diameter is 15mm with approx 1.3mm copper wire.

Just wondered if this coil is too small. After think about it again it might be all right.

Col
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Last edited by Firepower; Jul 13, 2012 at 09:33 PM.
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Old Jul 16, 2012, 09:19 AM
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12.5mm is fine. There is actually a very broad range of diameters that work for a helical antenna.

-Alex
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Old Jul 16, 2012, 09:52 AM
Firepower
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North Brisbane, Australia
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Thanks Alex,

I'll make it up.
Col


Quote:
Originally Posted by IBCrazy View Post
12.5mm is fine. There is actually a very broad range of diameters that work for a helical antenna.

-Alex
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Old Jul 16, 2012, 11:45 AM
Rotary~myPast,Present &Future!
SINGAPORE
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Col, from your first pic, your first quarter turn appears to be too far away from the reflector, I get the best results when the first quarter turn is 2mm away from and parallel to the reflector and is bent a fair bit upwards after the first quarter turn. I also get better results with a copper wavetrap than with an aluminium one.
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